1. Field of the Invention
The present invention relates to an ink composition. More specifically, the present invention relates to an ink composition, which has excellent storage stability and good defoaming properties, as well as improved resistance to bleeding on the print paper during image forming, while preventing nozzle clogging when used in a wet-type image forming apparatus. The invention is further directed to an ink cartridge containing the ink composition.
2. Description of the Related Art
Generally, colorants are substances which exhibit inherent colors by selectively absorbing or reflecting visible light. Colorants are generally divided into classes of dyes and pigments.
Dyes, which are dissolved in solvents, are colorants absorbed into materials to be dyed, such as fibers, leather, fur, and paper, by various methods. The dyes are selected to have considerable fastness even in conditions of constant daylight washing, friction, and the like. Pigments, which are insoluble in solvents, are particulates having coloring matters, and are not directly absorbed into the material to be dyed, but are adhered to the surface of the material to be dyed by physical methods (using, for example, adhesives or the like), to provide their inherent colors.
Ink compositions are used to color a specific object with a desired color by dissolving or dispersing a colorant in a solvent. Ink compositions are particularly suitable for forming images on paper. There are various types of ink composition according to the materials or methods used in preparation of the ink composition, such as ink compositions prepared by dissolving dyes in an aqueous or nonaqueous medium, ink compositions prepared by dispersing pigments in an aqueous or nonaqueous medium, or solid ink compositions which can be melted by applying heat.
Among these, ink compositions prepared by dissolving dyes in an aqueous medium are the most commonly used at present. Such ink compositions have excellent color tone and high stability when applied to the human body and the environment. Such ink compositions are used, for example, in a wet-type image forming apparatus, which are convenient for printing in the home or office and have become widely utilized in recent years. Accordingly, it is possible to form images of a high quality similar to the quality of a silver gelatin photograph.
The ink composition may be used in a wet-type image forming apparatus. To perform high-quality image recording over a long period using an aqueous ink composition for forming images, the ink composition should have suitable viscosity, surface tension, density, or other properties. If an inlet to the nozzle, which is the discharge port for the ink composition of a wet-type image forming apparatus, becomes clogged by used ink composition, if a precipitate is produced by heat or the like, or if the properties of the ink change during the storage of the ink composition, the quality of the image formed using the ink composition naturally deteriorates.
When the ink composition is stored for a long time in an ink storage part capable of storing a large amount of the ink composition, bubbles may be formed or dissolved in the ink composition. If the ink composition is used in this state, problems arise. The fluidity of the storage part or the flow passage of the ink composition may be inhibited, or bubbles may be absorbed into the nozzle, which has a narrow diameter, when the ink composition is ejected, so it may be difficult to eject the ink composition.
Furthermore, when printing at a high speed, that is, under high frequency operating conditions, the discharge performance of the ink composition is greatly affected by fine bubbles generated in the vicinity of the heater of the heat head, at the tip of the nozzles, or adhered to the sides of the nozzles. This effect has been increasingly discussed, because characteristics necessary for high-speed printing are important factors being demanded for the ink composition.
Therefore, it is necessary to remove the bubbles contained in the ink composition, or prevent the generation of bubbles.
U.S. Pat. No. 4,957,553 discloses an ink composition using a single defoaming agent in order to improve the defoaming properties of the ink composition while maintaining the surface tension of the colorant. In the same patent, secondary alkyl alcohols having 7 or less carbon atoms or ethylene oxide adducts thereof, acetylene alcohol or acetylene glycol, as a defoaming agent, are added to the ink composition. In this case, however, there are problems in that the nozzle is clogged, dynamic surface tension changes due to low solubility when the ink flows through the flow passage of the ink composition in a head of a wet-type image forming apparatus, and the antifoaming property may decrease depending on the concentration of the defoaming agent.
Additionally, U.S. Pat. No. 6,749,675 discloses an ink composition for a wet-type image forming apparatus in which one or more types of acetylene alcohol and one or more types of surfactant are used in order to improve recording stability.
A mixture of acetylene alcohol and surfactant separates into different phases when the fluid moves, so that long-term storage stability is reduced due to low fluidity and solubility of the acetylene alcohol.
A method is required to guarantee the performance of the additive, which prevents the generation of bubbles in the ink composition and removes the generated bubbles, without causing problems such as clogging the nozzle, reducing the antifoaming properties due to the concentration of the defoaming agent and phase-separation when the fluid moves.